Roughly equivalent to "clause" in English, except that the emphasis is on seeing this part of a sentence as needing completion, either with a second membrum (or colon) or with two others forming a tricolon. When membra (or cola) are of equal length, they form isocolon.
Membrum is also best understood in terms of differing speeds of style that depend upon the length of the elements of a sentence. The Ad Herennium author contrasts the slower speed of concatenated membra to the quicker speed of words joined together without conjunction (articulus).
Each of the three membra in the following sentence is numbered: